A number of techniques for stably directing a directional antenna towards a target under a rocking condition have been proposed mainly for use in an aircraft. In addition, since the beginning of the 1970's, a number of proposals aiming at the applications mainly for use in a marine equipment have been known. This is believed to have resulted partly from the encouragement by the fact that the MARISAT system for the maritime satellite was planned and executed in the United States. One of the features required for an antenna system for use in maritime satellite communication is that improvements in the electrical reliability as well as mechanical reliability must be provided. Since there is a tendency that this requirement be fulfilled by increasing a number of shafts in an antenna support device to reduce a movable range and a momentum for each axis, an axis assembly including three or more axes is being predominantly employed. In the case of a 4-axis assembly, the structure in which a stabilized platform is formed and an azimuth axis and an elevation axis are provided thereon, is easy to construct and is also highly practicable.
With regard to the method for forming a stabilized platform, a servo stabilization system has been heretofore known, in which a tilt angle is detected by an attitude sensor such as an inclinometer, a vertical gyro device, etc., a servo loop is assembled by combining with a servo control motor and stabilization is achieved by means of an angular velocity sensor such rate gyros, a gas rate sensor, etc. Since the beginning of the 1970's, a passive stabilization system employing gyros and a pendulum weight in combination has been proposed.
However, these two representative stabilization systems also have both an advantage and a disadvantage. In the system consisting of the combination of an attitude sensor and a servo control system, the cost of the attitude sensor and the stabilizing rate sensor is never small. Also in the passive system consisting of the combination of gyros and a pendulum weight, when the system is equipped on a vessel subjected to rocking of large amplitudes or equipped at a place remote from a center of rolling of a vessel such as a place on a mast, it is difficult to realize a compromise between the stability and the recovery to the horizontal state upon tilting of the stabilized platform.
Assuming that a horizontal stabilized platform has been supported so as to be rotatable about two axes, a description will now be made of external disturbances against the horizontal stabilized platform, by way of representative example, in connection with the case where the horizontal stabilized platform is mounted on a vessel. As representative disturbances, frictional torques transmitted from the two axes, i.e., the roll axis and pitch axis of the stabilized platform, and torques induced by linear accelerations in the case where the center of gravity of the horizontal stabilized platform is deviated, are known. As the linear accelerations, one can enumerate a linear acceleration caused by rolling or pitching, that caused by a water hammer, a gravitational acceleration, an acceleration in the direction of navigation of the vessel when it is accelerated or decelerated, vibrations caused by rotations of an engine or a screw, and the like. The accelerations caused by ordinary start, stop, acceleration and deceleration of a vessel can be neglected in most cases in a well-designed stabilized platform because they are sufficiently small.
Generally speaking, the undesirable external disturbances are torques induced by the frictions of the axes of the horizontal stabilized platform and by the linear accelerations of the rolling and pitching, water hammers and vibrations of the hull due to rotations of an engine and a screw of the vessel. These influences are generally emphasized when the antenna system is equipped on a mast or the like. These linear accelerations may act upon the horizontal stabilized platform as harmful external disturbance torques if the stabilized platform is not designed properly.
For instance, in the case of constructing the stabilized platform from the combination of gyros and a pendulum weight, when the above-mentioned various lateral linear accelerations A are exerted, the external disturbance torque exerted upon the stabilized platform is approximately equal to mAl, where m represents the mass of the pendulum weight and l represents the position of the center of gravity of the pendulum weight relative to the axis of the stabilized platform, and in some cases this torque would not be negligible.
Representing the sum of the external disturbance torques exerted upon one axis of the stabilized platform by N, then the stabilized platform precesses at the following angular velocity .OMEGA. about the other axis due to a gyro effect: EQU N=.OMEGA..times.H (1)
where H represents an angular momentum vector about a spin axis of a gyro. When the stabilized platform is approximately in a horizontal state, the three vectors in Equation (1) above are substantially at right angles to each other, and hence they can be represented by scalar quantities, as follows: EQU N=.OMEGA..multidot.H (2)
Accordingly, the following relation is derived: EQU .OMEGA.=(N/H) (3)
If the lateral linear acceleration is represented by A, then the torque N exerted upon the pendulum weight is derived by the following equation: EQU N.apprxeq.mAl+T.sub.f ( 4)
where T.sub.f represents a frictional torque exerted upon the same axis. Therefore, the angular velcocity .OMEGA. is represented by the following equation: EQU .OMEGA.=(N/H).apprxeq.(mAl/H)+(T.sub.f /H) (5)
In the case where the tilt of the horizontal stabilized platform is small, one way consider that the precession occurs at the angular velocity .OMEGA. represented by Equation (5) above.
It is to be noted here that the first term on the right side of Equation (5) is proportional to the linear acceleration A as well as the product m.multidot.l of the pendulum weight. This means that if the product m.multidot.l of the pendulum weight is large, when the linear acceleration is large, the external disturbance torque and thus the angular velocity of the precession also become large nearly in proportion thereto, and therefore, the large product m.multidot.l is undesirable because there is a fear that the platform may become unstable due to the large accelerations caused by a large water hammer applied to the hull, vibrations of the mast and the like. On the contrary, if the product m.multidot.l of the pendulum weight for damping is made small, a damping effect for the stabilized platform becomes weak, and hence the recovery of the stabilized platform are degraded.